Method for determining a vehicle reference velocity

ABSTRACT

A velocity and acceleration are measured ( 14 ) in a method for determining a vehicle reference velocity, said vehicle reference velocity being used for determining and selecting a suitable transmission ratio. If a measured acceleration (a ist ) exceeds ( 16 ) a maximally possible acceleration (a max ), a vehicle reference acceleration (a ref ) is determined depending on said exceeding. For this purpose, an acceleration correction value (BKF) is calculated ( 22 ) by means of which acceleration correction value a vehicle reference acceleration is calculated ( 24 ). The vehicle reference velocity (v ref ) is determined ( 26 ) by means of the vehicle reference acceleration (a ref ). A gearbox transmission ratio is determined ( 20 ) by means of the determined vehicle reference velocity. This branch of the procedure represents the case in which at least one wheel equipped with sensors slips.

FIELD OF THE INVENTION

The invention relates to a method for determining a vehicle referencevelocity of a motor vehicle, said reference velocity being used forselecting a suitable transmission ratio of a gearbox of the motorvehicle.

BACKGROUND INFORMATION

When automatically selecting a gearbox transmission ratio, the velocityof the vehicle over the ground is to be taken into consideration. Forthis purpose, sensors for measuring the velocity or the acceleration areattached to one or more wheels of the vehicle. A velocity measureddirectly or determined by integrating a measured acceleration and, inthe event that sensors are attached to several wheels, representing amean velocity is used as a vehicle reference velocity for selecting thetransmission ratio of a gearbox. In the event of slip of at least onewheel, the prior-art methods do not provide a suitable vehicle referencevelocity for selecting the transmission ratio of the gearbox any more.

Wheel slip is also taken into consideration with antilock brakingcontrol systems. Such systems use various mathematical manipulations forestimating the velocity of a vehicle for comparison with the rotationalvelocity of a given vehicle wheel for calculating a wheel slip of thiswheel, wherein the wheel slip is a parameter. DE-T2 689 08 007 describesa method for estimating a reference velocity or reference accelerationby periodic extrapolation. However, this method does not provide asuitable vehicle reference velocity for selecting the transmission ratioof the gearbox.

SUMMARY OF THE INVENTION INVENTION

It is therefore the object of the invention to provide a method fordetermining a vehicle reference velocity of a motor vehicle so that inthe event of wheel slip, even with simultaneous macro slip of allwheels, a vehicle reference velocity is provided that can be used forselecting a suitable transmission ratio of a gearbox of the motorvehicle.

This object is achieved according to the invention as disclosed andclaimed herein.

The invention relates to a method for selecting a suitable gearboxtransmission ratio in which a vehicle reference velocity (v_(ref)) isdetermined by comparing the actual acceleration with a maximallypossible acceleration (a_(max)). For this purpose, a velocity andacceleration are measured, wherein the velocity may be measured by meansof acceleration sensors and integrating measured acceleration data overthe time.

The next step is a check whether the measured acceleration (a_(ist))exceeds the maximally possible acceleration (a_(max)). The maximallypossible acceleration (a_(max)) is preferably determined on theassumption that the real friction between the wheel and the road is onits highest possible level.

If the measured acceleration (a_(ist)) does not exceed the maximallypossible acceleration (a_(max)), the vehicle reference velocity(v_(ref)) is determined by using the measured values.

If the measured acceleration (a_(ist)) exceeds the maximally possibleacceleration (a_(max)), a vehicle reference acceleration (a_(ref)) isdetermined depending on the extent to which the measured acceleration(a_(ist)) exceeds the maximally possible acceleration (a_(max)), whereinthe vehicle reference velocity (v_(ref)) is determined by means of thevehicle reference acceleration (a_(ref)).

The vehicle reference acceleration (a_(ref)) is preferably determined bymeans of an acceleration correction value (BKF). This accelerationcorrection value (BKF) is advantageously calculated byBKF=(a_(ist)−a_(max)) / (a_(top)−a_(max)), wherein a_(top) is a maximalvehicle-specific acceleration determined on the assumption that thefriction between the wheel and the road is on its lowest possible level.

Thus, the vehicle reference acceleration is preferably calculated bya_(ref)=(1−BKF)*(a_(max)−a_(min))+a_(min), wherein a_(min) is a minimalvehicle-specific acceleration determined on the assumption that thefriction between the wheel and the road is infinite.

The vehicle reference velocity (v_(ref)) is preferably determined byintegrating the vehicle reference acceleration (a_(ref)) over the time.

According to a further advantageous realization of the invention, theprocedure is repeated while the vehicle is moving.

The vehicle reference velocity (v_(ref)) is preferably used fordetermining a gearbox transmission ratio.

A motor vehicle according to the present invention has an automatictransmission controlled by the method for determining a vehiclereference velocity (v_(ref)) and particularly has an appropriatelyconfigured control device/automatic control device.

BRIEF DESCRIPTION OF THE DRAWINGS

Further advantages and details of the invention are set forth in thefollowing description of preferred exemplary embodiments on the basis ofthe drawing.

FIG. 1 shows a flow chart of a method according to the invention.

DETAILED DESCRIPTION OF AN EXAMPLE EMBODIMENT OF THE INVENTION

In FIG. 1, the flow chart 10 shows a method for determining a vehiclereference velocity according to the invention.

The procedure starts in step 12 that is followed by step 14 where avelocity and acceleration are measured. Step 16 is a check whether themeasured acceleration (a_(ist)) exceeds a maximally possibleacceleration (a_(max)).

If the measured acceleration (a_(ist)) does not exceed the maximallypossible acceleration (a_(max))(a_(ist)<a_(max)), the vehicle referencevelocity is determined in step 18 by using the measured values. Agearbox transmission ratio is determined in step 20 by means of thedetermined vehicle reference velocity. This branch of the procedurerepresents the case in which no wheel, or at least no wheel equippedwith sensors, slips.

If the measured acceleration (a_(ist)) exceeds the maximally possibleacceleration (a_(max)) in step 16 (a_(ist)>=a_(max)), an accelerationcorrection value (BKF) is calculated in step 22 byBKF=(a_(ist)−a_(max))/(a_(top)−a_(max)), wherein a_(top) is a maximalvehicle-specific acceleration determined on the assumption that thefriction between the wheel and the road is on its lowest possible level.By means of said acceleration correction value BKF, a vehicle referenceacceleration is calculated in step 24 bya_(ref)=(1−BKF*(a_(max)−a_(min))+a_(min), wherein a_(min) is a minimalvehicle-specific acceleration determined on the assumption that thefriction between the wheel and the road is infinite.

The vehicle reference velocity (v_(ref)) is determined in step 26 bymeans of the vehicle reference acceleration (a_(ref)) and preferably byintegrating the vehicle reference acceleration (a_(ref)) over the time.A gearbox transmission ratio is determined in step 20 by means of thedetermined vehicle reference velocity. That branch of the procedurerepresents the case in which at least one wheel equipped with sensorsslips.

Moreover, the procedure is preferably repeated while the vehicle ismoving. In this manner, a correct gearbox transmission ratio cancontinuously be determined while the vehicle is moving.

LIST OF REFERENCE NUMERALS

-   10 flow chart-   12 start-   14 measuring a velocity and acceleration-   16 a_(ist)>=a_(max)?-   18 determining the vehicle reference velocity by using the measured    values-   20 determining a gearbox transmission ratio-   22 BKF=(a_(ist)−a_(max))/(a_(top)−a_(max))-   24 a_(ref)=(1−BKF)*(a_(max)−a_(min))+a_(min)-   26 determining the vehicle reference velocity v_(ref) (a_(ref))

1. A method of determining a vehicle reference velocity (V_(ref)) of amotor vehicle that includes a vehicle wheel and that drives along atravel surface with friction between said vehicle wheel and said travelsurface, said method comprising the steps: a) providing a maximallypossible acceleration (a_(max)); b) determining (14) a measured velocityand a measured acceleration (a_(ist)) of said vehicle; c) checking (16)whether said measured acceleration (a_(ist)) exceeds said maximallypossible acceleration (a_(max)); d) if said measured acceleration(a_(ist)) does not exceed said maximally possible acceleration(a_(max)), then determining (18) said vehicle reference velocity(V_(ref)) as being said measured velocity; and e) if said measuredacceleration (a_(ist)) exceeds said maximally possible acceleration(a_(max)), then determining (24) a vehicle reference acceleration(a_(ref)) as:a _(ref)=(1−BKF)*(a _(max) −a _(min))+a _(min)  wherein: BKF=(a_(ist)−a_(max))/(a_(top)−a_(max)), a_(ref) is said vehicle referenceacceleration, a_(max) is said maximally possible acceleration, a_(min)is a minimal vehicle-specific acceleration based on said friction beinginfinite, a_(ist) is said measured acceleration, a_(top) is a maximalvehicle-specific acceleration based on said friction being at a lowestpossible friction value, and determining (26) said vehicle referencevelocity (V_(ref)) by integrating said vehicle reference acceleration(a_(ref)) over time.
 2. The method according to claim 1, furthercomprising controlling a system of said motor vehicle dependent on andresponsive to said vehicle reference velocity (V_(ref)).
 3. The methodaccording to claim 1, further comprising controlling a drive train ofsaid motor vehicle dependent on and responsive to said vehicle referencevelocity (V_(ref)).
 4. The method according to claim 1, furthercomprising determining and selecting a transmission ratio of atransmission of said vehicle dependent on and responsive to said vehiclereference velocity (V_(ref)).
 5. The method according to claim 1,wherein said step of determining said measured velocity comprisesmeasuring said measured acceleration and integrating said measuredacceleration over time to provide said measured velocity.
 6. The methodaccording to claim 1, wherein said step of providing said maximallypossible acceleration (a_(max)) comprises specifying said maximallypossible acceleration (a_(max)) for a condition that said friction is ata highest possible friction value.
 7. The method according to claim 1,further comprising repeating said steps b), c), d) and e) while saidvehicle drives along said travel surface.
 8. A motor vehicle comprising:a vehicle wheel adapted to rotate along a travel surface with frictionbetween said vehicle wheel and said travel surface as said vehicletravels over said travel surface; an automatic transmission; means formeasuring a measured velocity and a measured acceleration of saidvehicle; means for checking whether the measured acceleration exceeds amaximally possible acceleration; means for determining a vehiclereference acceleration (a_(ref)) as:a _(ref)=(1−BKF)*(a _(max) −a _(min))+a _(min)  wherein:BKF=(a_(ist)−a_(max))/(a_(top)−a_(max)), a_(ref) is said vehiclereference acceleration, a_(max) is said maximally possible acceleration,a_(min) is a minimal vehicle-specific acceleration based on saidfriction being infinite, a_(ist) is said measured acceleration, a_(top)is a maximal vehicle-specific acceleration based on said friction beingat a lowest possible friction value, if the measured accelerationexceeds the maximally possible acceleration; means for determining thevehicle reference velocity as being the measured velocity if themeasured acceleration does not exceed the maximally possibleacceleration, and for determining the vehicle reference velocity byintegrating the vehicle reference acceleration over time if the measuredacceleration exceeds the maximally possible acceleration; and means forcontrolling the automatic transmission dependent on and responsive tothe vehicle reference velocity.